Method of manufacturing a hollow section, grid stiffened panel

ABSTRACT

A method of manufacturing a hollow section grid-stiffened panel comprises providing a tool ( 10 ) having a surface ( 12 ). The stiffened skin composite panel is preassembled comprising laminating a composite outer skin ( 14 ) on the surface, placing a separator outer layer ( 16 ) on the composite outer skin ( 14 ), and laminating a composite stiffener ( 20 ) on the mandrel ( 18 ), the mandrel being positioned on the separator layer ( 16 ), wherein the separator layer ( 16 ) separates the stiffener ( 20 ) and the mandrel ( 18 ) from the outer composite skin ( 14 ). The preassembled outer skin composite panel is cured on the tool. The separator layer ( 16 ) and mandrel ( 18 ) are removed from the preassembled stiffened skin composite panel. The stiffened skin composite panel is reassembled, comprising applying an adhesive between the composite outer skin ( 14 ) and the composite stiffener ( 20 ). The reassembled stiffener skin composite panel is cured on the tool to bond the stiffener skin to the outer skin.

This application claims the benefit of provisional application No.60/281,984 filed Apr. 6, 2001.

FIELD OF THE INVENTION

The invention is directed to manufacture of stiffened skin compositepanels and, more particularly, to a method of manufacturing a hollowsection, grid stiffened panel.

BACKGROUND OF THE INVENTION

Stiffened skin composite panels are frequently used to replace compositeskin, honeycomb sandwich panels used for fairings and close-out panelson aircraft or other lightweight structures. One problem with using astiffened skin composite panel to replace a sandwich panel is weight. Ina conventional method of manufacturing a stiffened skin composite panel,structural foam is used as a mandrel over which stiffeners arelaminated. The foam mandrel generally remains in place in the finishedpanel. Inclusion of the foam mandrel results in overall panel weightthat exceeds that of a comparable performance sandwich panel. Thisincreased weight is generally intolerable.

One alternative conventional approach to manufacturing a stiffened skincomposite panel which results in hollow stiffeners is to produce a toolon which the skin portion of the panel is laminated and another tool onwhich the stiffener portion of the panel is fabricated. Such tools areexpensive and this option for manufacturing is often not economicallyfeasible. Another problem with this approach is the fit between thecomponents after they are fabricated on separate tools. Raw materialvariability also exacerbates the problems in a production setting. Theseproblems tend to eliminate a stiffened skin composite panel concept inmany instances.

Another general problem with composite skin sandwich panel fairings isthe eventual accumulation of moisture in the honeycomb cells. Themoisture degrades the structural performance of the panels as well asincreases the weight of the assembly. Hollow stiffeners with drain holesprevent the accumulation of moisture and all of the associated problems.

The present invention is directed to overcoming one or more of theproblems discussed above, in a novel and simple manner.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a method ofmanufacturing a stiffened skin composite panel.

Broadly, the method comprises providing a tool having a surface. Astiffened skin composite panel is preassembled comprising laminating acomposite outer skin on the surface, placing a separator layer on thecomposite outer skin, and laminating a composite stiffener on themandrel, the mandrel being positioned on the separator layer, whereinthe separator layer separates the stiffener and mandrel from thecomposite outer skin. The preassembled stiffened skin composite panel iscured on the tool. The separator layer and mandrel are removed from thepreassembled stiffened skin composite panel. The stiffened skincomposite panel is reassembled, comprising applying an adhesive betweenthe composite outer skin and the composite stiffener. The reassembledstiffened skin composite panel is cured on the tool to bond thestiffener to the outer skin.

Alternatively, the method comprises manufacturing a hollow section, gridstiffened panel, comprising providing a tool having a surface. The gridstiffened composite panel is preassembled, comprising laminating acomposite outer skin on the surface, placing a separator layer on thecomposite outer skin, and laminating a composite grid stiffener on agrid shaped mandrel, the mandrel being positioned on the separatorlayer, wherein the separator layer separates the grid stiffener and themandrel from the composite outer skin. The preassembled stiffened skincomposite panel is cured on the tool. The separator layer is removed.The mandrel is removed from the preassembled stiffened skin compositepanel. The grid stiffened skin composite panel is reassembled,comprising applying an adhesive between the composite outer skin and thecomposite grid stiffener to provide a hollow cross section defined bythe composite outer skin and the composite grid stiffener. Thereassembled stiffened skin composite panel is cured on the tool to bondthe grid stiffener to the outer skin.

Further features and advantages of the invention will be readilyapparent from the specification and from the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a first operation for a method ofmanufacturing a stiffened skin composite panel comprising preassemblingthe stiffened skin composite panel;

FIG. 2 is a partial, cut away perspective view illustrating thepreassembled stiffened skin panel;

FIG. 3 is an exploded view, similar to FIG. 1, illustrating reassemblingthe stiffened skin composite panel;

FIG. 4 is a perspective view, similar to FIG. 2, illustrating thereassembled stiffened skin composite panel;

FIG. 5 is a perspective view illustrating the reassembled stiffened skincomposite panel ready for a bond curing operation;

FIG. 6 is a perspective view of the stiffened skin composite panelmanufactured according to the method of the present invention; and

FIG. 7 is a cross sectional view illustrating a portion of the stiffenedskin composite panel.

DETAILED DESCRIPTION OF THE INVENTION

As described in greater detail below, a stiffened skin panel structureuses a silicon rubber mandrel on which stiffeners are laminated with therubber mandrel positioned on a previously laminated outer skin. Aseparator layer is laid up between the skin and the stiffener laminates.Once the assembly is “cured” in an oven or autoclave, then the parts areseparated and subsequently bonded together at the split line.

The method in accordance with the invention generally comprisespreassembling a grid stiffened skin composite panel on a tool, thencocuring the preassembled stiffened composite panel on the tool,removing a separator layer and a mandrel used in the preassembling step,reassembling the grid stiffened skin composite panel and curing a bondline in the reassembled stiffened skin composite panel.

The described method is advantageously used for forming a composite skinpanel used on the wing of an airplane, also referred to as a “fairing.”The method can also be used for manufacturing other types of compositepanels that might be used on aircraft or other lightweight structures.The method produces a hollow section, grid stiffened panel to provide alightweight structure having sufficient strength while saving on toolingcosts. The latter feature is accomplished by using a single tool forboth the preassembling and reassembling of the grid stiffened compositepanel during the manufacturing process along with a rubber mandrel andseparator layer, as described more particularly below.

Referring initially to FIG. 1, the preassembling step is generallyillustrated and described. A tool in the form of a mold 10 has a surface12. The surface 12 defines the desired shape of the composite panel tobe produced. As such, the surface 12 could be planar, or could becontoured, as shown, as necessary for the composite panel to beproduced.

A composite outer skin 14 is laid up on the tool surface 12. This stepconventionally comprises laying up, or laminating, the required numberof face sheet plies on the tool surface 12. Most typically, the outerskin 14 comprises laminates of fiber reinforced plastic. A fabric ispre-impregnated with resin which will bond the laminate during asubsequent curing operation.

A separator layer 16 is placed on the laminated composite outer skin 14.Generally, the separator layer 16 comprises a material that would notadhere to resin. Particularly, the separator layer 16 is of a material,which may be coated, selected so that it does not adhere to the otherlayers or melt. Polyethylene or Teflon® film may be used as theseparator layer 16. Additionally, the separator layer 16 should leave noresidue that would inhibit bonding in subsequent operations.

A mandrel 18 is oriented and fixed on the separator layer 16. Theparticular shape of the mandrel 18 defines the grid configuration andhollow cross section to be produced. In the illustrated embodiment ofthe invention, the mandrel 18 comprises a rubber mandrel, such as ofsilicon rubber. The mandrel 18 is precured and will not melt at curingtemperatures to be used. When used with a contoured tool surface 12, themandrel 18 is flexible to take the shape of the contoured surface 12. Inthe illustrated embodiment of the invention, the mandrel 18 includesangled walls to provide a trapezoidal configuration in cross section.While not required, the trapezoidal configuration facilitates removaland renders it easier to consolidate stiffener laminates.

A stiffener laminate 20 is next laid up on the rubber mandrel 18. Thestiffener laminate is formed of pre-impregnated fiber material (wovenfabric or uni-directional), similar to the outer skin 14. In theillustrated embodiment of the invention, the stiffener laminate 20resembles a single piece structure having a defined shape correspondingto that of the grid shaped mandrel 18. This is its finished appearanceremoved from the mandrel 18. As is known to those skilled in the art,the stiffener laminate 20 is laid up by laminating strips of fiberreinforced plastic over the grid structure of the mandrel 18 in twodirections to leave openings corresponding to openings in the grid ofthe mandrel 18. The drawing illustrates the stiffener laminate 20 afterlay up is completed.

Prior to cocuring the preassembled stiffened skin composite panel, aformed rubber caul sheet 22 is placed over the stiffened skin compositepanel and sealed to the mold 10 in a conventional manner. The caul sheet22 is flexible and may be formed of silicon rubber approximately ⅛ inchthick. It is formed to resemble the profile of the rubber mandrel 18.

A resultant preassembled stiffened skin composite panel 23 isillustrated in greater detail in FIG. 2, with parts cut away forclarity. Specifically, the outer skin 14 is laminated on the toolsurface 12. The separator layer 16 is on the composite outer skin 14.The composite stiffener 20 is laminated on the mandrel 18, the mandrel18 being positioned on the separator layer 16. As a result, theseparator layer 16 separates the stiffener 20 and mandrel 18 from thecomposite outer skin 14.

This preassembled stiffened skin composite panel 23 is cocured in anoven, an autoclave or a free standing cure with autoclave pressure,vacuum pressure, mechanical pressure or ambiant conditions, in aconventional manner. The heat and pressure applied to the preassembledstiffened skin composite panel 23 during this cocuring operation causesthe outer skin 14 and stiffener 20 to come into proper shape. The resinin the laminates bonds with fabric in the laminates to form a unitaryouter skin 14 and a separate, unitary stiffener 20.

Subsequently, the caul sheet 22 is removed from the tool 10. Next, thestiffener laminate 20 and rubber mandrel 18 are removed from atop theseparator layer 16. The separator layer 16 is then peeled from the outerskin 14. After individually inspecting the components, an adhesive layer24, see FIG. 3, is applied between the outer skin 14 and the stiffener20. In the illustrated embodiment of the invention, the adhesive 24 isillustrated as an adhesive ply having a configuration corresponding tothat of the stiffener 20. Alternatively, the adhesive 24 could be aliquid adhesive applied to either the outer skin 14 or the stiffener 20,or both, as necessary or desired. Next, the stiffener 20 is reassembledonto the outer skin 14 in the same position and orientation as in thefirst operation described above relative to FIG. 1. A formed rubber caulsheet 26, which is generally similar to the rubber caul sheet 22 of FIG.1, is then placed over the reassembled stiffened skin composite panel 28on the tool surface 12.

The resultant reassembled stiffened skin composite panel 28 isillustrated in greater detail in FIG. 4, with parts cut away forclarity. Specifically, the outer skin 14 is positioned on the toolsurface 12. The adhesive layer 24 is disposed between the compositestiffener 20 and the composite outer skin 14. The caul sheet 26 coversthe reassembled stiffened skin composite panel 28.

The reassembled stiffened skin composite panel 28 is then curedaccording to recommended cure cycles for the materials being used. FIG.5 illustrates the reassembled stiffened skin composite panel 28 on thetool 10 covered by the rubber caul sheet 26 ready for the bonding cure.The curing operation bonds the grid stiffener 20 to the outer skin 14using the adhesive 24.

Using the single mold tool 10 and the single cure operation for thestiffener 20 and outer skin 14, in combination with the removablemandrel, provides a unique fit between the stiffener 20 and skin 14 toproduce the reassembled stiffened skin composite panel 28, see FIG. 6.

An exemplary cross section of the reassembled stiffened skin compositepanel 28 is illustrated in FIG. 7. In the exemplary embodiment, theouter skin 14 comprises three plies of fabric. Similarly, the stiffener20 comprises three plies of fabric with two layer of tape also includedon an outer most wall 30. As is apparent, the stiffener 20 on the outerskin 14 comprises a hollow section 32 which is trapezoidal in crosssection corresponding to the cross section of the mandrel 18, asdiscussed above.

The configuration of FIG. 7 utilizes hat-section stiffeners on a meshgrid network. The stiffener cross section of the pattern of thestiffener grid is optional and can be structurally optimized in adetailed design of a particular fairing or close out panel.

The adhesive 24 provides a bond at an interface between the stiffener 20and outer skin 14. In the first curing operation, the interface is onlyseparated by the separator layer 16. Thus, the interface produced in thepreassembly and first curing operation is the same interface used in thereassembly step and second curing operation to provide a unique fit forthe hollow section, grid stiffened panel 28 manufactured as described.Particularly, any potential fit problems between components areeliminated since the parts are cured together but the mating interfaceis only separated by the separator layer 16.

The resulting overall panel 28 has a weight comparable to that of ahoneycomb sandwich stiffened panel by virtue of having hollowstiffeners. Drain holes can easily be added to the hollow stiffeners topreclude the deleterious effects of moisture accumulation.

The method described herein drastically reduces up-front tooling costssince the rubber mandrels can be economically fabricated and areconformable to required panel curvatures or contours.

1. The method of manufacturing a stiffened skin composite panel,comprising: providing a tool having a surface; preassembling thestiffened skin composite panel, comprising placing a composite outerskin on the surface, placing a separator layer on the composite outerskin, and placing a composite stiffener on a mandrel, the mandrel beingpositioned on the separator layer, wherein the separator layer separatesthe stiffener and mandrel from the composite outer skin; cocuring thepreassembled stiffened skin composite panel on the tool; removing theseparator layer and the mandrel from the preassembled stiffened skincomposite panel; reassembling the stiffened skin composite panel,comprising applying an adhesive between the composite outer skin and thecomposite stiffener; and curing the reassembled stiffened skin compositepanel on the tool to bond the stiffener to the outer skin.
 2. The methodof claim 1 wherein the composite outer skin and the composite stiffenercomprise fiber reinforced plastic.
 3. The method of claim 1 wherein theseparator layer is selected from polyethylene orpolytetrafluoroethylene.
 4. The method of claim 1 wherein the mandrelcomprises a silicon rubber mandrel.
 5. The method of claim 1 whereincocuring the preassembled stiffened skin composite panel and curing thereassembled stiffened skin composite panel both comprise using anautoclave, an oven or a free standing cure with autoclave pressure,vacuum pressure, mechanical pressure or ambient conditions.
 6. Themethod of claim 1 wherein the cocuring and curing steps compriseproviding a caul sheet sealed to the tool.
 7. The method of claim 1wherein the tool surface has a contoured surface.
 8. The method of claim1 wherein the composite outer skin and the composite stiffener compriseresin impregnated fabric.
 9. The method of claim 8 wherein the separatorlayer is formed of a material that does not adhere to the resin.
 10. Themethod of claim 1 wherein the mandrel comprises a formed rubber mandrel.11. The method of claim 10 wherein the mandrel has a trapezoidal crosssection.
 12. The method of claim 1 wherein cocuring the preassembledstiffened skin composite panel comprises cocuring the compositestiffener and the composite outer skin having an interface onlyseparated by the separator layer.
 13. The method of claim 12 whereincuring the reassembled stiffened skin composite panel comprises theadhesive providing a bond at the interface.
 14. The method ofmanufacturing a hollow section, grid stiffened panel, comprising:providing a tool having a surface; preassembling the grid stiffened skincomposite panel, comprising placing a composite outer skin on thesurface, placing a separator layer on the composite outer skin, andplacing a composite grid stiffener on a grid shaped mandrel, the mandrelbeing positioned on the separator layer, wherein the separator layerseparates the grid stiffener and mandrel from the composite outer skin;cocuring the preassembled stiffened skin composite panel on the tool;removing the separator layer; removing the mandrel from the preassembledstiffened skin composite panel; reassembling the grid stiffened skincomposite panel, comprising applying an adhesive between the compositeouter skin and the composite grid stiffener to provide a hollow crosssection defined by the composite outer skin and the composite gridstiffener; and curing the reassembled grid stiffened skin compositepanel on the tool to bond the grid stiffener to the outer skin.
 15. Themethod of claim 14 wherein the composite outer skin and the compositegrid stiffener comprise fiber reinforced plastic.
 16. The method ofclaim 14 wherein the separator layer is selected from polyethylene orpolytetrafluoroethylene.
 17. The method of claim 14 wherein the mandrelcomprises a silicon rubber mandrel.
 18. The method of claim 14 whereincocuring the preassembled grid stiffened skin composite panel and curingthe reassembled grid stiffened skin composite panel both comprise usingan autoclave, an oven or a free standing cure with autoclave pressure,vacuum pressure, mechanical pressure or ambient conditions.
 19. Themethod of claim 14 wherein the cocuring and curing steps compriseproviding a formed caul sheet sealed to the tool.
 20. The method ofclaim 14 wherein the tool surface has a contoured surface.
 21. Themethod of claim 14 wherein the composite outer skin and the compositegrid stiffener comprise resin impregnated fabric.
 22. The method ofclaim 21 wherein the separator layer is formed of a material that doesnot adhere to the resin.
 23. The method of claim 14 wherein the mandrelcomprises a formed rubber mandrel.
 24. The method of claim 23 whereinthe mandrel has a trapezoidal cross section.
 25. The method of claim 14wherein cocuring the preassembled grid stiffened skin composite panelcomprises cocuring the composite grid stiffener and the composite outerskin having an interface only separated by the separator layer.
 26. Themethod of claim 25 wherein curing the reassembled grid stiffened skincomposite panel comprises the adhesive providing a bond at theinterface.